Making SPST switch into SPDT with PNP transistor

Discussion in 'General Electronics Chat' started by mattbrailsford, Oct 31, 2015.

  1. mattbrailsford

    Thread Starter New Member

    Oct 31, 2015
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    Hey All,

    I need to make a SPST momentary switch act like a SPDT switch so thought I could use a PNP transistor for the job. I'm just struggling to understand from my research the correct setup.

    I've mocked up a circuit in the falstad circuit editor (link here) which seems to work:

    [​IMG]

    The load I'll be using is different but I've just added an LED + Resistor as POC. The thing I'm unsure of though is if I should need any additional resistors or something around the PNP? All the examples I see have a resistor on the base pin, but I'm not sure if this tends to be more for microcontroller use? I'll be using a physical switch.

    Any advice is much appreciated.

    Cheers

    Matt
     
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Your PNP transistor appears to be an n-channel FET. If you draw your schematic in a conventional orientation it will be a lot easier to follow and yes, in most cases, transistors require base resistors.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    That's not a common symbol for a MOSFET, but it does represents a P-MOSFET.
     
  4. mattbrailsford

    Thread Starter New Member

    Oct 31, 2015
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    Sorry, my bad, I did mean a p-channel MOSFET rather than a PNP transistor, which is actually what is used in the circuit diagram. Sorry it's not the best diagram, I'm just not that good at doing them. Hopefully it'll come with experience, but I've tried to make it as simple as I could.

    Matt

    PS I tried to edit my main post to change PNP to p-channel MOSFET but can't seem to see how you edit them?
     
  5. blocco a spirale

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    Oops, but it is drawn funny:)
     
  6. dl324

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    Mar 30, 2015
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    First off, as has already been mentioned, your schematic is difficult to follow.

    Secondly, it can't be working because you have no latch element; so as soon as the momentary switch is released, circuit reverts to the previous state.

    What kind of a load are you planning to switch?
     
  7. mattbrailsford

    Thread Starter New Member

    Oct 31, 2015
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    Sorry, I don't know any other way.

    That's the behavior I want, I've only used a latching switch in the falstad demo so I can explicitly watch both states. In reality, the circuit will on enable when a momentary switch is heald down, on release, I want it to go back to original state.

    This circuit is part of a bigger one, where in normal "released" mode, the power runs two elements, an LED and sends voltage through a 200k potentiality to be measured by an IC. When pressed, the current goes through a different LED, and also through a fixed 100k resistor to the same pin as the pot on the IC for measuring. Ultimately, when open, a green LED should be on, and the IC reads a variable resistance from the pot, and when closed, a red LED should be on, and the IC reads a fixed resistance from the 100k resistor. This is all on 3v.

    Hope that helps.

    Matt
     
  8. mattbrailsford

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    Oct 31, 2015
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    [Deleted duplicate post]
     
  9. dl324

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    Flow should be right-to-left and top-to-bottom as much as possible. Components should be drawn upright, unless doing otherwise makes sense.

    EDIT: Of course I meant left-to-right, not right-to-left...
    upload_2015-10-31_13-3-13.png

    In your original post you said
    An SPDT switch has 2 positions.
     
    Last edited: Oct 31, 2015
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    With the post #9 circuit, when the switch is open LED 2 will be on. Is that what you want?
     
  11. mattbrailsford

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    Oct 31, 2015
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    Ahh, thanks for the pointer. Much appreciated.

    I must be missing something. A SPDT switch has 2 positions right? NO and NC? So I'm trying to replicate it using the mosfet. When the switch is released, one circuit completes, and when the switch is pressed, the original one cuts, and the other one completes, as if one was wired to the NO position and the other wired to the NC position.

    Ohhh, sorry, it's because I didn't say "SPDT momentrary switch" isn't it? My bad. I don't need latching, I just need to perform two tasks, one while released (open), and one wile pressed (closed).
     
    Last edited: Oct 31, 2015
  12. mattbrailsford

    Thread Starter New Member

    Oct 31, 2015
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    Yes, and when switch is closed, LED 2 is off, and LED 1 is on.

    I'm just not sure if it requires any pull up / down resistors anywhere? or one on the gate pin as I've seen in some schematics.
     
  13. dl324

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    Unless I made a transcription error, that's what the OP drew in his first post. I was just giving an example of how schematics are usually drawn.
     
  14. dl324

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    A "momentary" or "push button" switch has contacts labeled normally opened and normally closed; but might not have both.

    SPST would be ON or OFF.
    SPDT would be ON-ON.
    SPDT with center off would be ON-OFF-ON.

    A momentary SPST would be (mom) or OFF.
     
    Last edited: Oct 31, 2015
  15. mattbrailsford

    Thread Starter New Member

    Oct 31, 2015
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    Ahh, thanks for clarifying.

    If we ignore my misunderstanding of SPDT / SPDT then, does my explanation of what I'm after clarify what I'm aiming to achieve though? (I think it's probably much simpler than I'm explaining)
     
    Last edited: Oct 31, 2015
  16. dl324

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    If I understood what you're describing; no.

    Draw a block diagram of what you want.
     
  17. mattbrailsford

    Thread Starter New Member

    Oct 31, 2015
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    [​IMG]
     
  18. dl324

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    Focus on how you want to connect what you want to measure to whatever is measuring it.
     
  19. mattbrailsford

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    Oct 31, 2015
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    Sorry, I don't know what you mean?
     
  20. AnalogKid

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    Aug 1, 2013
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    The circuit in post #1 and clarified in post #9 will do what you have described. However, I would add a resistor from the Q1 gate to GND. You are relying on a trickle of current to through LED1 to turn off Q1. 10 K from Q1 gate to GND will give more stable performance.

    ak
     
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